A study on measuring the 222Rn in the Buriganga River and tap water of the megacity Dhaka

Radon (222Rn), an inert gas, is considered a silent killer due to its carcinogenic characteristics. Dhaka city is situated on the banks of the Buriganga River, which is regarded as the lifeline of Dhaka city because it serves as a significant source of the city’s water supply for domestic and industrial purposes. Thirty water samples (10 tap water from Dhaka city and 20 surface samples from the Buriganga River) were collected and analyzed using a RAD H2O accessory for 222Rn concentration. The average 222Rn concentration in tap and river water was 1.54 ± 0.38 Bq/L and 0.68 ± 0.29 Bq/L, respectively. All the values were found below the maximum contamination limit (MCL) of 11.1 Bq/L set by the USEPA, the WHO-recommended safe limit of 100 Bq/L, and the UNSCEAR suggested range of 4–40 Bq/L. The mean values of the total annual effective doses due to inhalation and ingestion were calculated to be 9.77 μSv/y and 4.29 μSv/y for tap water and river water, respectively. Although all these values were well below the permissible limit of 100 μSv/y proposed by WHO, they cannot be neglected because of the hazardous nature of 222Rn, especially considering their entry to the human body via inhalation and ingestion pathways. The obtained data may serve as a reference for future 222Rn-related works.


Introduction
Human beings are continuously exposed to natural radiation, mostly coming from terrestrial and extra-terrestrial sources (Rani et al., 2021). Among the existing sources of ionizing radiation in the environment, Radon ( 222 Rn) alone is the major contributor (more than 50%) of the total radiation dose to humans (UNSCEAR, 2010). 222 Rn is the only gaseous element in the 238 U decay series and possesses no color, odor, or taste. This ( 222 Rn) short-lived (T1/2=3.82 days) radioactive nucleus is formed due to the alpha decay of 226 Ra. Among the three naturally occurring radioisotopes of radon, 222 Rn is the most abundant in nature as Thoron ( 220 Rn) and Actinon ( 219 Rn) have relatively very short half-lives of 55s and 3.2s, respectively.
Radon is present naturally in the earth's strata. Its abundance in the earth's crust fluctuates with the variation of geology and lithology of the area. Due to its high mobility, radon gas can swiftly travel from soil and rocks to water and air. Albeit, the concentration of radon in water depends on the temperature, lithology, geology, rainfall, and earthquake activities (Faweya et al., 2021). 222 Rn is highly volatile, easily dissolved, and emanates from the water. Since safe water is essential for human lives, it is a matter of great concern to evaluate the radon level in the water sources such as rivers and tap water. A relatively higher concentration of radon is found in groundwater than in surface water due to the aeration process (Faweya et al., 2021). Because of its gaseous nature, 222 Rn is used as a tectonic tracer (Ahmad et al., 2019) to determine the tectonic fault lines and predict earthquakes.
Radon is considered a hazardous gas due to its potential to affect human cells and tissues biologically. Ingestion through the gastrointestinal tract and inhalation via the respiratory tract are the two major pathways of entering radon into the human body. Both pathways are potentially risky, affecting the lung and the gastrointestinal system. Although radon is chemically inactive with other substances and biological cells, it is the second most prominent cause of lung cancer after tobacco smoking (WHO 2009). In the case of inhalation, the short-lived metallic progeny of radon (mostly 218 Po and 214 Po) are deposited in the lungs and damage the cells and the tissues of the respiratory system via high-energy alpha emission.
That is why it is one of the main contributors to escalating lung cancer risks. The IARC (The International Agency for Research on Cancer), a part of WHO, classified radon as a group 1 carcinogen (Copes & Peterson, 2014;Ismail et al., 2021a).
Water is vital for all life; human beings use water regularly for various purposes, including bathing, drinking, etc. However, water consumption is the primary cause of radon exposure through the ingestion pathway, whereas the emanation of radon from water causes exposure through the inhalation of air. As radon is loosely soluble in water, it can easily emanate from water to air (Thumvijit et al., 2020a). For that reason, radon activity measurement in water is necessary to protect people from radiological hazards. Many international organizations propose a safe limit on radon concentration in water, and almost all developed countries have their national guidelines for radiation safety. The World Health Organization recommended a safe limit of 100 Bq/L for radon in the water (WHO, 2008a), whereas the USEPA suggested the maximum contamination level (MCL) of 11.1 Bq/L (US EPA, 2012). The USEPA also proposes an alternative maximum contamination level (AMCL) of 148 Bq/L (US EPA, 2012). To apprehend the health hazard of radon, measurement of the annual effective dose due to radon ingestion and inhalation is essential. The WHO and IAEA recommend that the total annual effective dose due to radon in water should be < 100 µSv(WHO, 2017).
Numerous studies have been performed worldwide to measure the radon level in various water resources such as tap water, river water, deep well water, bore well water, bottled water, etc. (Faweya et al., 2021;Mustapha et al., 2002;Rani et al., 2021;Thumvijit et al., 2020a;Yong et al., 2021). Almost every developed country has its national reference limit of radon in water and indoor air to ensure radiological safety for public health. Bangladesh has no such reference level for radon in water. In fact, to the best of our knowledge, no studies on radon measurement in surface water, tap water, or indoor air have yet been conducted. Millions of people living in the Dhaka megacity solely rely on tap water for their daily household purposes, such as washing, bathing, drinking, cooking, etc. Recent studies claim that tap water contains excessive heavy metals (Hossain et al., n.d.). The Buriganga river has already been heavily polluted, and recent studies show that the situation is getting worse day by day (Bhuiyan et al., 2015;Whitehead et al., 2019). Nevertheless, it meets around 20% of the total water demand of Dhaka city dwellers. In addition, it serves as one of the busiest major transportation routes/hubs, as well as many businesses and trade centers that are situated on the bank of this river. This indicates a greater possibility of radon exposure to the general populace. So, it is necessary to measure the radon level in the tap water and the Buriganga river water to find out if it is within the safe limit or not, which eventually will help to ensure the radiological safety of public health.
The purpose of this study is to (a) measure radon concentration in the chemically and biologically polluted Buriganga river water and the tap water of the megacity Dhaka, b) calculate the associated radiological hazards, c) to contribute to the setting up of a factual baseline data which will assist the authority to structure a national reference level of radon water.

Study Area
Dhaka, the capital city of Bangladesh, as well as one of the most densely populated megacities in the world, is the prime focus of this study. Dhaka is located between latitudes 23°42' and 23°54'N and longitudes 90°20' and 90°28'E. The geographical area of this city is 306.38 square kilometers, where more than 21 million people inhabit (Bangladesh Bureau of Statistics, 2015). Several rivers like Buriganga, Balu, Tongi Khal, and Turag surround the city from the south, east, west, and north (Dhaka, Geology -Banglapedia, n.d.), respectively.
However, the Buriganga river has a major share, and it forms the southern and western boundaries of Dhaka city. The length of this river flowing through Dhaka is 11 km, the depth is 10m, and the width is 400m. The latitude and the longitude of this river are 23° 37' 59.99" N, 90° 25' 59.99" E (Buriganga Riverkeeper : » History, n.d.). Because of the large-scale industrial activities on the bank of the Buriganga river, it has become the worst polluted river in the country.

Geology of Dhaka city and its periphery region
The megacity Dhaka is placed at the southern end of the Madhupur tract, 1.5-10 m (average 6 m) above the adjoining floodplains (Burgess et al., 2011). The area is characterized by Quaternary alluvial sequences of the Madhupur Tract, known as Pleistocene terrace deposits surrounding Holocene deposits of the peripheral rivers (Bodrud-Doza et al., 2020;Mohammad Abdul Hoque, 2004;Rahman et al., 2013). The geological map and the cross section along NS and EW of the study area are illustrated in Fig. 1(b) and Fig. 1(d On the other hand, the Lower Madhupur Clay deposits primarily contain pale yellowish to yellowish brown sandy clay to clayey sand and silty sand with similar nodules and spots but less weathered and oxidized than the upper (Burgess et al., 2011;Karim et al., 2019). The Holocene deposits are further subdivided into alluvial flood plain deposits comprising natural levee deposits, bar deposits, point bar deposits, back swamp deposits, flood plain deposits, and valley fill deposits. Flood plain deposits mainly comprise grey to dark grey colour sticky clay to clayey silt, with discontinuous sand, oxidized root, rootlets, and organic matter. The valley fill deposits consist of dark grey to yellowish to olive brown, silty clay, marshy clay, and peat (Karim et al., 2019). A sequence of fine to coarse-grained micaceous quartzofeldspathic sands containing Dupi Tila Formation of Pliocene age, hydro geologically known as the Dupi Tila aquifers, the primary aquifer of Dhaka city, underlies the Madhupur Clay and is not exposed anywhere in the city (Bodrud-Doza et al., 2020;Burgess et al., 2011;DWASA, 2008;Rahman et al., 2013). A gravel bed lies at the bottom of the Dupi Tila Formation, which grades upward from coarse-grained sands to medium-grained sands to fine-grained sands at the top. The Dupi Tila Formation is divided by a discontinuous clay layer into two aquifers: an upper fine-grained aquifer (approximately 40-50 m thick) and a lower coarse-grained aquifer (approximately 80 m thick) (Burgess et al., 2011). A summary of the Pliocene to Recent lithological and aquifer characteristics of the study area is depicted in Fig. 1(c). The geochemical study of the groundwater of Dupi Tila aquifer shows that the Ca/Mg-HCO3 type and weathering of aluminosilicates control the distribution of major ions in the aquifers (Islam et al., 2021). The Dupi Tila and Madhupur Formations are isolated by extensive incision of the land surface during the late Quaternary and forming several faults at their boundaries which affect the aquifer river system and the groundwater flow of this area (Bodrud-Doza et al., 2020;M Kamrul Hasan et al., 1999;Mohammad A Hoque et al., 2007;Rahman et al., 2013). It is assumed that due to the elevation of the river bed with the top of the Dupi Tila sands has through the connection between them and the rivers surrounding Dhaka (i.e., Buriganga, Balu, and Turag River) and the aquifer is possible along certain reaches (Ahmed & Burgess, 2003;Burgess et al., 2011;M K Hasan et al., 1998). Cross-section along the NS and EW direction in the study area.

Sampling:
Thirty water samples, including 20 river water and 10 tap water (Fig. 1a), were collected in November 2021 using a 500 mL plastic bottle prior to the winter season. The river water samples were collected from the highly polluted Buriganga river by following the stratified sampling technique approved by IAEA (International Atomic Energy Agency, 2019). The majority of the samples were collected from heavily populated river bank areas such as Sadarghat, Showari Ghat, Mitford ghat, Gabtoli, etc. The bottle was fully submerged directly into the water during the river water collection to prevent air bubbles in the bottle. The tap waters were collected from different localities of the megacity Dhaka using a systematic grid sampling technique approved by the IAEA (International Atomic Energy Agency, 2019).
Before sample collection, the tap was opened for several minutes, and the water was allowed to flow. Afterward, the bottle was filled and sealed tightly. Prevention of aeration during sampling was the prime concern to avoid the escape of dissolved radon in the water. Each of the samples was labeled with a unique sample ID (RW for river water and TW for tap water), and the GPS of the collection points and the collection time were recorded. These water samples were taken immediately to the Laboratory of the Health Physics Division in Atomic Energy Centre Dhaka of Bangladesh Atomic Energy Commission.

Experimental Procedure:
The radon activity concentration in collected water samples was measured using RAD7, a portable electric radon detector with RAD-H2O accessories (manufactured by Durridge Co. Ltd). A schematic diagram of the experimental setup is illustrated in Fig. 2. The inner cell of the RAD7 is a hemisphere coated with an electrical conductor where the energy of emitted alpha particles from radon and its progeny are converted into electrical signals. Before analyzing the samples, the RAD7 detector needs to be radon free and dry. Dry air was purged for 10 minutes, lowering the humidity below 10%. The collected water samples were transferred into a 250 mL glass vial and connected with the RAD7. The radon emanation occurred by aerating the water via a glass frit in a closed-loop system. An internal air circulating pump recirculates the air through the closed-loop system to extract the radon from the water until the equilibrium is reached. The wat-250 process was selected to measure radon in water, where the extraction efficiency was 94%. The equilibrium state is reached within 5 minutes, and after this, no more radon can be extracted from the water. The air is circulated by the pump aerating the water and supplying the radon to the RAD7 detector. This process runs for 30 minutes in four cycles to measure the radon in the samples. The RAD7 summarizes the average and corrected radon concentration measurements obtained from each sample for four cycles at the end of the run in a printout.

Dosimetry Calculation
Internal radon exposure comes primarily from radon inhalation and ingestion, which is harmful to the respiratory organs. When water is collected and used, radon is inhaled, and radon is ingested when radon-contaminated water is consumed. Therefore, by using Equations (1) and (2), the annual effective dose due to radon inhalation and ingestion is calculated from the experimentally measured values of the radon concentration (Ismail et al., 2021a;Rani et al., 2021;Thumvijit et al., 2020b;UNSCEAR, 2010).
Where, ∑ and ∑ represents effective doses due to ingestion and inhalation, respectively

Radon in river water:
As demonstrated in Table 1, the measured radon concentration in the collected twenty river water samples from the highly polluted Buriganga river varied from 0.349 ± 0.180 to 1.160 ± 0.610 Bq/L with an average of 0.675 ± 0.285Bq/L. The maximum radon concentration (1.160 ± 0.610 Bq/L) was found in the sample collected from the Forashgonj Kheyaghat area (RW17). There is a direct swage-drain line (from the Dolai Khal) near the Forashgonj Kheyaghat, which may contaminate the area with technologically enhanced naturally occurring radioactive materials (TENORMs), consequently may increase the radon level in that location.
The sample collected from the Wais Ghat area (RW15) had the minimum radon concentration (0.349 ± 0.180 Bq/L). The radon level in these river water samples is relatively low as the aeration of surface water accelerates the emanation of radon into the environment (Ismail et al., 2021b;Orosun et al., 2021). No sample either contained a radon concentration level more than the safe limit of 100 Bq/L recommended by the WHO or exceeded the maximum contamination limit (MCL) of 11.1 Bq/L set by USEPA (Al Zabadi et al., 2012a;Şahin Bal et al., 2021;States, 1999;Yong et al., 2021).
For each river water sample, the annual dose due to radon inhalation and ingestion is listed in Table 1. The mean annual effective dose due to river water ingestion and inhalation were 1.725 µSv/y and 1.701 µSv/y, respectively. The total annual effective dose for river water ranged from 1.771 µSv/y to 5.887 µSv/y with an average of 3.426 µSv/y. All of these values were well below the maximum permissible limit of 100 µSv/y set by WHO (Darabi Fard et al., 2020;Ismail et al., 2021a;Kareem et al., 2020;WHO, 2008b). In Table 2, the present study for river water is compared with the reported results worldwide. The radon level was found very high in some river water, such as the radon level (60 Bq/L) in the Rajouri of Pir Panjal, Kashmir was high due to the mountainous area where many minerals were found in the soil of that region (Nazir et al., 2020). The study at Ekiti, Nigeria, claimed that the high radon level (42-88 Bq/L) was found in river water due to the local geology covered with migmatite, porphyritic granite, granite gneiss, and undifferentiated schist (Faweya et al., 2021). In another study, the authors claimed the Gold and Bismuth mining site near the study area in Edu LGA, Kwara State, Nigeria, was the main reason for the high radon level (19.14 ± 3.98 Bq/L) (Lawal, 2021). Nevertheless, the geological map of the Buriganga shows that there are no mountains or volcanic areas around this river. Neither any mining site nor the study area was covered with minerals. The Buriganga riverbed is mainly clay instead of rocks (Dhaka, Geology -Banglapedia, n.d.). These were the significant reasons for the low radon level in this river water. Additionally, the result of this study is consistent with the previous research carried out in different regions of the world, such as in India (Rajashekara et al., 2007;Shivanandappa & Yerol, 2018), Iraq (Kareem et al., 2020), and Malaysia (Ismail et al., 2021b).  (Ismail et al., 2021b) Kwara,Nigeria 15.97 (Orosun et al., 2021) Ekiti, Nigeria 42.22 to 88.22 (Faweya et al., 2021) Edu, Nigeria 19.14 ± 3.98 (Lawal, 2021) Punjab, India 3.37 ± 0.29 (Rani et al., 2021) Rajouri, Pir Panjal 60 (Nazir et al., 2020) Kirkuk, Iraq 0.359 (Kareem et al., 2020) Hemavathi River, India 0.67 (Shivanandappa & Yerol, 2018) Transylvania, Romania 0.9 to 4.5 (Nita et al., 2013) Karnataka, India 0.16 to 1.79 (Rajashekara et al., 2007) Dhaka, Bangladesh 0.675 ± 0.285 Present work

Radon in tap water:
As illustrated in Table 3, the radon concentration in the ten tap water samples collected from Dhaka city varied from 0.559 ± 0.302 to 3.060 ± 0.605 Bq/L with an average of 1.537 ± 0.380 Bq/L. The lowest radon concentration (0.559 ± 0.302 Bq/L) was found in the Bongshal area (TW10). The sample collected from the Mohammadpur area (TW04) contained the highest radon concentration (3.060 ± 0.605 Bq/L). A thorough investigation found that deep tube well water was stored in a tank and then supplied to the tap in the house from where the TW04 was collected. The water was stored in a closed tank that prevented air contact with water. For this reason, the radon gas hardly emanates from the water, so the radon level was higher than the others. However, all the samples contained lower radon levels than both the maximum contamination limit (MCL) of 11.1 Bq/L set by USEPA and the safe limit of 100 Bq/L recommended by the WHO (Al Zabadi et al., 2012a;Şahin Bal et al., 2021;States, 1999;Yong et al., 2021).
The annual effective dose due to radon inhalation and ingestion for each tap water sample is listed in Table 3. The maximum and the minimum values of annual effective dose due to tap water ingestion were 7.818 µSv/y and 1.428 µSv/y, with an average of 3.928 µSv/y.
For inhalation, it ranged from 1.409 µSv/y to 7.711µSv/y with a mean of 3.874 µSv/y. The total annual effective dose for tap water ranged from 15.530 µSv/y and 2.837 µSv/y with an average of 3.932 µSv/y. All of these values were way below the maximum permissible limit of 100 µSv/y set by WHO (Darabi Fard et al., 2020;Ismail et al., 2021a;Kareem et al., 2020;WHO, 2008b).  Table 4 compares the present study for tap water with the reported literature worldwide.
According to the previous literature, high radon level in tap water was found in some countries.
A study in the Sabzevaran fault, Iran, found the radon level in tap water higher (17.12 Bq/L) than in the MCL. The authors concluded that a high radon level was due to volcanic, metamorphic, and sedimentary rocks surrounding the study area (Shamsaddini et al., 2020). A study in Kenya found the radon level (37 Bq/L) much higher than the MCL in tap water samples; due to the studied area being located near a volcanic region, and the maximum tap water of the area was collected from underground water sources, the local geology was the primary reason for the abnormally higher radon level (Mustapha et al., 2002).  (Mittal et al., 2016) Kedah, Malaysia 7.0 ± 0.71 (Ahmad et al., 2015) Bihor, Romania 6.9 (Moldovan et al., 2014) Nablus, Palestine 1.0 (Al Zabadi et al., 2012b) Bursa, Turkey 0.91 to 12.58 (Akar Tarim et al., 2012) Kenya 37 (Mustapha et al., 2002) Dhaka, Bangladesh 1.537 ± 0.380 Present work The tap water of Dhaka city is collected from surface water treatment plants as well as extracted the underground water by using different pumps (Urban Water Blueprint - Dhaka, n.d.), which are then supplied all over the city through a piping system. However, the geology of the present study area neither consisted of any volcanic, granitic, or metamorphic rock nor any volcanic region nearby. These may be the leading causes of the lower radon level in the tap water of Dhaka city. Moreover, the result of this study is consistent with many studies conducted in China (Yong et al., 2021), Thailand (Thumvijit et al., 2020a), Palestine (Al Zabadi et al., 2012b), Malaysia (Salih, 2021), India (Yashaswini et al., 2020).
The present study shows that the radon level in river water is much lower than in tap water. River water is easily in contact with the open air, which accelerates the emanation of radon, while tap water has less contact with the air. Tap water is supplied in a closed piping system from the storage tank to the tap, and so the aeration is negligible compared to surface water. Additionally, a portion of the tap water of Dhaka city is supplied from a groundwater source which was the primary reason for the high radon level in some samples like TW04.
According to the Stochastic model of radiation, there is no threshold limit; even a single atom can cause severe damage to the body, so a slight amount of radon in water should be a concern.
Even though all of the measured values of the radon level in the water and the associated effective dose were below the USEPA and the WHO limit, a radon mitigation treatment plant should be introduced. The water should be aerated and boiled before consumption, reducing the radon concentration in the water. No national reference level has yet been set for radon in water, so this study being the first radon-related work in Dhaka city, may contribute to forming a national safety limit for radon in water.

Conclusion
measured using a RAD H2O detector to ensure public health safety from radiological hazards.
The ranges of measured radon concentrations in the river water (0.349 ± 0.180 to 1.160 ± 0.610 Bq/L) and the tap water (0.559 ± 0.302 to 3.060 ± 0.605 Bq/L) showed lower than the limit set by the WHO and the USEPA (States, 1999). Also, the total annual effective doses were within the safe limit set by the WHO (WHO, 2008b).
Considering the carcinogenic characteristics of 222 Rn, a radon mitigation program should be introduced, and frequent monitoring of radon in various dwelling media is essential to ensure the safety of public health. Further extensive research should be carried out for radon mapping of the country. Moreover, this study may serve as factual reference data to set a national reference radon limit which is currently absent in the country.